In designing planar antennas for use in wireless communication systems, the typical goals set are to achieve powerful performance with low structural profiles, low costs of manufacture, ease of manufacture, and ease of integration with other communication devices. However, conventional planar antennas such as microstrip patch antennas and basic types of planar inverted-L or -F antennas (ILA or IFA) have inherent narrow impedance bandwidths, which typically are of measures of a few percent. This drawback adversely affects the usefulness of these conventional planar antennas in broadband applications. Therefore, many techniques have been proposed for alleviating the narrow impedance bandwidth problem.
For microstrip patch antennas, the proposals typically include the addition of parasitic elements, the use of electrically thick substrates, or the introduction of matching networks. For the planar ILAs or IFAs, the proposals typically include replacing wire radiators with planar radiators and/or loading the planar antennas with high permittivity material.
The techniques proposed for alleviating the narrow impedance bandwidth problem have drawbacks. Adding parasitic elements vertically or laterally to microstrip patch antennas increases the sizes, costs and complexity of manufacture of such planar antennas. Using electrically thick substrates in microstrip patch antennas increases the costs of manufacture and lowers the radiation efficiency of such planar antennas due to the increased surface waves and dielectric loss. Introducing matching networks to microstrip patch antennas reduces the radiation efficiency and complicates the design and fabrication of these planar antennas. The ILAs or IFAs usually have low polarization purity and are therefore not suitable for applications requiring purely polarized waves, for example in polarization diversity applications. The planar ILAs or IFAs that are loaded with materials of high permittivity have large sizes and involve high costs of manufacture.
In a number of articles, a type of suspended plate antennas is proposed to further improve impedance bandwidths for such planar antennas. The articles include: T. Huynh and K. F. Lee's “Single-layer single patch wideband microstrip antenna,” Electronics Letters, vol.31, pp.1310-1312, 1995; N. Herscovici's “A wide-band single-layer patch antenna,” IEEE Trans. Antennas and Propagat., vol.46, pp.471-473, 1998; and K. M. Luk, C. L. Mak, Y. L. Chow, and K. F. Lee's “Broadband microstrip antenna,” Electronics Letters, vol. 34, pp.1442-1443, 1998. The proposed suspended plate antennas are placed at a height of approximately 0.1 times the operating wavelength above a ground plane. A variety of matching techniques is introduced to these planar antennas for realising good matching conditions in broadband applications. The ameliorated impedance bandwidth typically is of a measure ranging from 10% to 40% for signals at 2:1 voltage standing wave ratio (VSWR).
In Table 1, measurements relating to the critical performances of three types of conventional low-profile planar antennas are tabulated for comparison. The suspended plate antenna is shown to be more suited for broadband applications.
TABLE 1Comparison of critical performance measures of conventionalplanar antennas with low profilesPolarizationAntennaEfficiencyBandwidthPuritySize/CostMicrostrip patchLow<10%GoodFair (lowantennasprofile)/highInverted L- or F-High<10%—Fair (lowantennasprofile)/lowSuspended plateHigh10˜40%BadFair (lowantennas(>−15 dB)profile)/low
The proposed suspended plate antennas greatly alleviate the narrow impedance bandwidth problem, usually fed by probe-type feeds because a variety of matching techniques has been used to realise good matching conditions for such planar antennas. However, the undesirable higher-order modes and the asymmetric feeding schemes result in seriously degraded radiation performance of these planar antennas. The high cross-polarization levels and the distorted radiation patterns to a great extent limit practical applications of the suspended plate antennas, where planar antennas of high polarization purity, such as arrays and polarization diversity designs, are required. For example, dual-polarization base stations usually require planar antennas with high polarization purity. This drawback therefore severely limits the scope of practical applications of broadband suspended plate antennas.
Techniques are therefore proposed for compensating the degraded radiation performance. A number of articles (Z. N. Chen and M. Y. W. Chia's “Broadband probe-fed plate antenna,” 30th European Microwave Conference, Paris, France, vol.2, pp.182-185, October 2000; and Z. N. Chen and M. Y. W. Chia's “Broadband rectangular slotted plate antenna,” Proc. IEEE Antennas and Propagat. Symp., Slat Lake City, Utah, USA, vol. 2, pp.640-643, July 2000) proposed the replacement of U-shaped slots in such planar antennas with Ω-shaped slots or narrow notches. Compared to suspended plate antennas that have slots with U-shapes or large aspect ratios, suspended plate antennas with Ω-shaped slots or narrow notches have lower cross-polarization levels. This is because the effect of such slots on current distributions at the plate radiators is reduced and as a result the higher order modes are to some degree suppressed. However, the cross-polarization levels of suspended plate antennas with Ω-shaped slots are still high although such levels have been lowered by about 2 dB when compared with suspended plate antennas with U-shaped slots, since the higher order modes are not suppressed completely and the plate radiators are still fed asymmetrically.
In other articles (P. S. Hall's “Probe compensation in thick microstrip patches,” Electronics Letters, vol. 23, pp.606-607, 1987; and A. Petosa, A. Ittipiboon, and N. Gagnon's “Suppression of unwanted probe radiation in wideband probe-fed microstrip patches,” Electronics Letters, vol. 35, pp.355-357, 1999), dual-feeding structures are proposed for use on such planar antennas to ease the serious degraded radiation performance of these planar antennas. A pair of probes with a phase shift of 180° is located symmetrically about the mid-line of radiators. The demand of a 180° phase shift leads to the implementation of a complex feeding network because to design such a broadband phase shifter is difficult. Additionally, this proposal leads to the lowering of only the cross-polarization levels in the H-plane.
There is therefore a need for a broadband suspended plate antenna with a feeding structure and a method therefor, which alleviates cross-polarization levels in the H-planes and distorted radiation patterns in the E-planes.